By Lorraine Orlandi

MEXICO CITY, Jan 12 (Reuters) - A Mexican rights activist arrested on what supporters said were false charges engineered by powerful businessmen was suddenly released on Thursday as international pressure mounted on his behalf.

A surprised and somewhat giddy Martin Barrios, 33, walked out of jail in central Mexico's Puebla state, apparently exonerated of attempted blackmail charges brought last month by the head of a local textile factory where Barrios had helped workers file labor complaints.

"I don't really have any idea what happened, they just forced me to leave," Barrios told Reuters with a laugh during an interview by cell phone moments after leaving the jail.

"The government just threw in the towel," he added. "I said, I didn't do anything to the man (his accuser) so how can they pardon me?"

Family members said it appeared the charges had been dropped, but the details were unclear. Government officials could not be reached for comment.

Activists said it was a rare victory for justice in a nation where the police and courts are routinely used by special interests to silence opponents, despite a promise by President Vicente Fox to clean up a shoddy rights record and overhaul the justice system.

International and local groups including Amnesty International and the AFL-CIO labor organization ratcheted up pressure on Puebla's government in recent days to release Barrios, a father of two, using public demonstrations and local media coverage.

"What's obvious is that the government took license that it shouldn't have, to go after rights activists in Mexico, and underestimated the power of the people," said Ben Cokelet of the Solidarity Center in Mexico, an affiliate of the AFL-CIO.

He and other rights leaders want charges dropped in a related case against journalist Lydia Cacho, who is accused by Puebla textile industry leader Kamel Nacif of defamation.

Barrios, a member of Puebla's human rights commission, was accused of attempted blackmail by Lucio Gil, the owner of a garment factory in the state and a subcontractor of Nacif.

In November, Barrios helped workers file a labor complaint against Gil, who agreed to resolve the issues, according to the Washington Office on Latin America rights group. Gil did not comply and fired the 163 workers involved, a violation yet to be redressed, the group said.